1. Field of the Invention
The present invention relates to a weighing scale in which a load cell is used to detect and measure the value of a weight loaded on the weighing tray having a mechanical connection with the load cell.
2. Description of the Prior Art
Load cells for use in weighing scales have conventionally been of the Roberval type for the purpose of eliminating an error arising from an eccentric or uneven loading of weight on the weighing tray. Load cells of this type are described, for instance, in U.S. Pat. Nos. 4,107,985 and 4,143,727. For eliminating such an error, however, a precisely parallelogrammatic form of the load cell must be ensured, so that a highly excellent workmanship is required for making a load cell of the Roberval type. Further, the load cell of the Roberval type, which is typically shown both in FIG. 1 as a perspective view and in FIG. 2 as a cross-sectional view taken along the A--A plane indicated in FIG. 1, must be subjected to a corrective processing by chipping off the corner parts shown in black in FIG. 2 for making strain gauges S as weight detectors produce the same weight signal irrespective of the position of weight loading on the tray D. In practice the chipping is carried out so that a constant weight value can be obtained even if the point of weight loading is varied from W1 to W5 on the tray D. This corrective work, which can not help being made manually by a skilled hand, increases the cost of manufacturing the scale. In case of a scale whose weighing capacity is small, the work of correction is very difficult, because the parts subject to the correction are very thin. In addition the load cell of the Roberval type can not be made shorter than a certain height on account of the requirements both for ensuring an exact parallelogrammatic form of the load cell and for eliminating the errors arising from the tension, compression and distortion produced in the cross-sectional planes of gauge-applied parts by an eccentric or uneven weight loading on the weighing tray. Therefore, it also is a disadvantage that the Roberval type load cell makes it difficult to assemble a scale of low height. A further definitive disadvantage of the Roberval type is that an adhesive or moisture-proofing coating to fix the strain gauges affects the accuracy and sensitivity of the load cell, especially in case of a scale of a small weighing capacity. For instance, in case of a scale with a weighing capacity of 600 gr., the gauge-applied parts of the load cell become one millimeter thick or thinner. A highly accurate scale can hardly be constituted with a load cell of the Roberval type.